This invention relates to audio amplifiers, and more particularly to elimination of noises from such an audio amplifier.
In general, noises caused by an electrical semiconductor element are shot noise caused by the irregular motion of carriers passing through the p-n junction, a thermal noise due to the channel resistance or the base diffusion resistance, and a 1/f noise cause by the recombination, in the base surface, of carriers injected from the emitter and by the leak current in the collector junction, for instance. It is known in the art that the electromotive force (or current) caused by such a noise depends on the magnitude of a signal applied to or injected into the element, and the probability of the electromotive force which reaches a certain noise electromotive force assumes a Gaussian distribution.
A conventional audio amplifier is so designed as to select an operating point at which the noise factor at a predetermined input resistance thereof is minimum. However, as noises caused upon arrival of a signal are not periodic signals, it is difficult to sufficiently apply feedback thereto, and the noises caused in the active elements are amplified and outputted by these active elements. It is considered that this difficulty accompanying the conventional audio amplifier is due to the following reason. That is, upon arrival of an AC audio signal the circuit is not balanced in the resulting AC mode of operation although it is balanced by proper biasing in its "no-signal" or DC mode. In other words, heretofore a reference by which the circuit should be balanced (in an AC mode) for a signal applied thereto has not been analyzed.